December 14, 2016

This post will describe the reasoning behind
our general interest, and the eventual use of very heavy sled (VHS) sprint
training in our recent study. This project (and all the associated references)
was designed following our review of literature on the topic of sled training
for sprint performance, and the ideas of two Master students I’ve had the
pleasure of supervising this year: Matt Cross (Auckland University of
Technology) and Satya Vesseron (University of Nice). In the interest of
maintaining ease of reading I won’t list all the references used to support my
statements - however they are all available in the papers mentioned above for
those of you who want to dig deeper into the science behind VHS.

Two blog posts by George Petrakos (Glasgow
Warriors) have tackled the issue pretty well:

As you can read in these posts and the present
one, by VHS we mean sled loading of more than 30% of body mass (BM), up to
loads around 100-120%BM… you should give it a try, just for fun. By the way,
even if we should express resistance as a friction force (see the recent work
of Matt Cross here), for more clarity and comparison to the extisting
literature, we’ll express “load” as the total mass pulled by the athlete (sled
mass + additional mass).

Even if my personal background and passion is
athletics and sports training, I think that one method of innovation in this
field is not to design hypotheses and studies according to what is or has been
done in the field. Even if very successful coaches have brought some innovative
training methods and still do, my point as a researcher is to start from the
theoretical side of things, and ask the question: “what does make sense,
mechanically speaking?” Regardless of whether or not this is what is actually
done in practice, including at very high levels.

Innovation is not doing what has always been
done better, it is doing something else. Something that makes more sense,
rather than simply following tradition.

The traditional way of using sleds (at least
in the scientific literature and for most coaches especially in athletics) is
to keep the load comparatively light (typically 10-15%BM, or ~10% decrement in
maximum velocity) so that the maximal running speed reached at the end of the
acceleration is not altered too much, and the overall running pattern is
maintained. The associated argument is something like: “If sled training
induces a slow running speed and modifies the running pattern and technique, it
is therefore non-specific; and non-specific is not good”. By such a
logic, only free sprinting should be used to train for sprint performance, and consequently
any other exercise (e.g. gym- or weightlifting-based) should be judged as
non-specific and thus counterproductive. My point here is that there should be
a separation between specific and non-specific exercises, based on the training
goal. That is, we consider VHS as a strength and conditioning exercise, not a
sprint-specific exercise.

The most interesting part of this project is
how it all started. A few weeks after publication of our review in Sports
Medicine - which showed that almost no studies had tested the effect of VHS
training - a Master student approaches me and says: “Prof., I am a soccer
coach, and I think VHS could be a very good training stimulus for my amateur
players”. Wow. Except for my lessons about force-velocity profiling in
sprinting, this student had not read the above-mentioned blog posts or our
review. Before giving him my opinion, I asked him: “why do you think so?”. He
answered that basically, his players had no access to a gym to work on their
lower limb strength, and no time for specific training sessions to work on
transferring that strength into a better sprint acceleration technique and
performance. His main argument was that VHS is both a cost- and time-effective way
of overloading the athlete, training both lower limb strength (i.e. general
capacity) and the technical ability to apply this force effectively into the
ground (i.e. horizontally-oriented force). After all, the only tools required
are a solid sled and harness, 100kg of weightlifting plates, and access to a
field for sprinting.

At the same time, another Masters student I
was supervising (Matt Cross from Auckland) was working on a very similar topic,
and his preliminary findings were showing that much heavier sprints than
currently recommended showed promise for the development of maximal power. The
discussions with these two students led to the following biomechanical
arguments in favor of VHS, regardless of the traditional thoughts of most
coaches:

Argument 1: When
sprinting maximally against a VHS you still move damn slowly!

Haha nice one, my 7-yr old daughter knows it,
no PhD needed here. But the point here is not that you run slow, it’s that
because you run slow, your horizontal force output is much higher than when you
run faster (according to the force-velocity curve, applied to sprinting). For
full details on this F-V thing, please check our recent synthesis. So VHS, more
than light sleds and much more than unloaded sprinting, is theoretically a way of
targeting development of the “force” side of your horizontal F-V profile. That’s
how human muscles work, too much speed = not much force, and vice versa. In
fact, our understanding of how much load is required to overload these
capacities is not very well understood – as we’ll discuss later in this post. So
if my F-V profile shows low maximal force output (F0), light sleds or unloaded
sprinting is likely a poor way of developing these capacities. Furthermore, it
makes sense to expect that the higher the load, the higher the force output
during acceleration, as shown in loaded jumping, cycle sprinting, or treadmill
sprinting.

Argument 2: Sprinting
with a VHS allows you to incline your body and push forward much more, compared
with lighter sleds or during unresisted sprinting.

The higher the resistance, the greater the
possible forward orientation, and vice versa. If we assume a good relationship
between the overall incline of the body and the orientation of the resultant
ground reaction force (GRF) vector, VHS is a very useful way of providing this
incline. As an example, in the picture below the athlete is pulling a load
equivalent to ~120%BM. There is no way he could apply force onto the ground
with such a horizontally-oriented angle of propulsion with lighter loads.

Athlete pulling a load equal to 120% of his own body mass...definitely forward-oriented push

Argument 3:
hip extensors work

Although no scientific evidence has been brought
to our knowledge, our practice, anecdotal evidence, and athlete’s comments tend
to show that hip extensors, mainly glutes, are particularly stimulated by the
VHS modality. It makes sense mechanically (more crouched position, higher load
against lower limb extension). Almost systematically, subjects who started to
familiarize with VHS reported muscle pain after their first training sessions
(glutes especially, hamstring to a lesser extent). In the video below, a PhD
student I’m supervising performed a full session of VHS (about 10 sprints of
20-m in total, using loads of 40 to 100%BM). Our phone conversation 2 days
after this session was fun: Although he is practicing weightlifting and
resistance training, he suffered major DOMS at the glutes…

Argument 4: When
sprinting with a VHS you push slowly (thus hard) and forward for a longer
period.

This is the main argument behind VHS in my
opinion, and combines the previous 3 points: Sprinting using VHS allows you to
create and maintain conditions
of high force, high forward lean, and high muscular activity (anecdotal), only
seen for single instances during unloaded sprinting, for an extended period of
time. As seen in the figure below, during an unresisted sprint acceleration,
maximal power is typically reached within the first 2 seconds (for a single
footstrike), and everything that happens afterward is a
low-force/low-power/high-speed exercise, in the horizontal direction. In
addition, during unresisted sprinting and with a lighter load (or a parachute),
the body cannot stay forward inclined for a long time so you are forced to get
vertical pretty quickly. As you can see in the video above, in a VHS context, you can apply more horizontally-oriented
GRF, for a much longer period/distance. The result is the accumulated cumulated
overload per sprint is much higher than using unresisted or lightly loaded sprinting
exercise.

Our computation of Usain Bolt's velocity, horizontal force and power outputs during the World Record acceleration phase

VHS training is not popular
in athletics, compared to light sled training. I’ve heard some coaches use very heavy resistances during pulling
exercises, in rugby for instance, but sled training has hitherto mostly been
performed with light loads (e.g. 10 to 20%BM at most). Some coaches even talk
about heavy loads when the total mass pulled is around 20kg, i.e. about 25% of
the athlete’s body mass. I guess one of the argument is that when using heavier
loads during this “sprint” exercise, the running speed is too much altered, and
one may not consider it a sprint exercise anymore. As far as we can tell, the
argument in the sports science academic literature originated with a couple ofstudies in the early to late 2000’s that tested changes in kinematics with
resisted sprinting loads and recommended that “The lighter load is likely best for use in a
training program“. Collectively, their results showed that the use of loads that induce
>10% decrease in running speed (approximately), significantly altered the
sprint running pattern (mainly joint kinematics). Consequently, these papers
suggested the ‘optimal’ loading for training was one that did not change acute
running technique, for fear that this would likely result in long term negative
adaptations in these ‘technical’ factors, and decrease performance. On this
basis, the result was a recommendation against heavy loads. For instance, a 2008 study using loads that induced a ~10% decrease in maximal velocity
reports: “The athlete should use a high load so
as to experience a large training stimulus, but not so high that the device
induces substantial changes in sprinting technique“. According to the citation record, these
articles appear to have steered the direction of research and practice, with
researchers following suit with these recommendations and generally steering
clear of VHS, or using tentatively ‘heavy loads’ of up to 42.6% BM – unsurprisingly,
those sticking to the ‘guidelines’ and selecting loading protocols
‘non-significantly different’ to unloaded sprinting have typically found no
differences between resisted sprinting and free sprinting training intervention
outcomes...

Our main point here is that VHS should not be
seen as an exercise ‘specific’ to unloaded sprinting, but instead it should be
seen as a horizontal strength or power based exercise that may result in
beneficial adaptations in determinant factors of sprint performance. The points
that follow were key in our approach towards experimentally testing the
hypothesis that VHS would be an effective training stimulus for
sprint-specific, horizontally-oriented force production.

Turning points…

The first trigger for this project was our publications showing the
importance of a forward-oriented GRF (mechanical effectiveness) for sprint
acceleration performance. We showed the importance of this mechanical
effectiveness in both low and elite level athletes in two cross-sectional
studies, but did not perform associated training studies to answer the basic
question: How can athletes develop their mechanical effectiveness and
horizontal force production for an improved acceleration performance?

The second trigger was the publication of two timely studies by Naoki
Kawamori and his colleagues. The first one showed that, during the second step
of a sprint acceleration, pulling a sled load of 30%BM (higher load than in
previous studies) induced a significantly higher ratio of force (the index of
mechanical effectiveness, computed as the ratio between the horizontal
component of the GRF and the resultant GRF over the step, see picture). The
ratio of force was 28% on average in the control (unresisted sprinting)
condition versus 31% in the “classical” 10% BM sled condition and 39% in the
30% BM condition… clearly a more effective ground push. In addition, no
difference in sprint kinetics were observed between the 10% BM condition and
the control condition, suggesting (as highlighted above) an insufficient
mechanical overload.

Kawamori et al.’s second paper was a training
intervention, that compared two groups training (8 weeks, 2 sessions/week) with
sled loads that induced 10% (light load, about 13%BM for these subjects) versus
30% (heavy load, about 43%BM) decrease in maximal sprint velocity. Although
mechanical effectiveness was not measured in this study, the conclusions were
clear: “Heavy- and light-load weighted sled
towing were equally effective in improving sprint acceleration ability over 10
m, but only the use of heavy load improved 5-m sprint performance. Therefore,
it is conclusive that coaches and athletes should abandon the myth regarding
the optimal training load of weighted sled towing (i.e., 10% rule) and should
explore the use of heavier external resistance for weighted sled towing.“

Our review of literature published in 2015 by
George Petrakos overall concluded that more studies were needed to clarify the
effects of sled training (especially heavy: 20-30%BM and very heavy: >30%BM loads)
on sprint acceleration performance. Note that we are talking about performance
(sprint time/speed) here, not sprint technique or running pattern. My opinion
is that what gives you an advantage in most sports (soccer, rugby, etc.) is
being fast, not having a “good” technique. By the way, what is ‘good technique’?
A technique that makes you fast, first and foremost. So if the training
intervention makes you faster, it is effective.

The last trigger was the Masters work of Matt Cross, which aimed to
assess the mechanics underlying resisted sled sprinting. The work showed that
not only did sled sprinting exist on the same Fv spectrum as unloaded sprinting
(confirming our initial ‘Argument 4’), but the loads that maximized horizontal
power output were ~70-100%BM. Note that based on these results, some
athletes may need to go higher than ~100%BM to work in the ‘force end’ of the
Fv spectrum! Obviously these loads are much greater than those previously studied. In fact, while not published in the thesis, our calculations
suggest that sprinting using this ‘optimal loading’ scheme presents an acute
response in power output ~3x greater than traditional ~10%BM loading.

So based on all these points, and according to
our theoretical framework around the sprint force-velocity-power profiling, we
hypothesized that VHS training would mainly result in improved mechanical
effectiveness and thus maximal horizontal force and power outputs. In our view
of individualized sprint training, VHS could be an effective way to train that
specific part of the profile, for athletes whose individual profile shows a
deficit in this particular part of the sprint.

"VHS" means Very Heavy Sled...

What we did, and what our
study says

Briefly (all details in the paper), 20 amateur
soccer players were assigned to a control group and a VHS group, after
familiarization with heavy loaded sled sprinting. Subsequently, each group
performed 2 sessions of ten 20-m sprints for 8 weeks. The control group
performed unresisted sprints, while the VHS group performed sprints against sled
loads of 80%BM (so about 60kg on average). Except for the 4 players in the
control group who could not perform the study or the post-testing (personal and professional issues), all players
completed the program safely. The results showed that overall, VHS was an
effective stimulus for improving mechanical effectiveness (i.e. more
horizontally-oriented GRF during the early phase of acceleration post-training)
and in turn the maximal horizontal force and power outputs. In addition, the
increase in sprint performance tended to be higher in the VHS group, especially
at 5m. The improvements in sprint performance for the control group were negligible.

The results from this pilot study basically
confirm the theoretical arguments listed above, and highlight the interest of
VHS training in this specific context.

For a cool video summary of this study, see below

What the study does not
say: limitations and future studies

Here is a list of arguments for those who
focus more on what studies did not do/say, unfortunately too many people out
there.

Mainly, this pilot study needs confirmation
and replication with other populations (higher level athletes in soccer or
other sports). Our subjects were amateur soccer players, so we don’t know if
this method will be effective and useful in other populations. There were not
many subjects involved, and we would hope for more statistical power in future studies
on the topic. The load, although heavy, was not great enough to work on the
maximal force area of the F-V curve, as Matt Cross’ work shows. Rather, these
athlete were working more in the zone of maximal power. Nevertheless, this provided
a sufficient overload to improve force output in these subjects. Other force
dominant cohorts, highly trained rugby players for instance, will likely
require much greater loading schemes (100%BM or more?). These subjects were not
trained for resistance or weightlifting, so they might have been good
responders to the VHS stimulus… well that was exactly our point! As highlighted
early in this post, part of the value of VHS is it provides a good mix between
strength and mechanical effectiveness stimuli.

What about the “10% rule”
and the alteration of the running “technique”

This study clearly shows the interest of VHS
training to improve mechanical effectiveness and horizontal force output during
the early acceleration phase. Although we did not directly compare our results
with those of 10%BM loads, our conclusion based on the existing literature is
that this 10%BM load is likely not heavy enough to induce such improvements. So
you may maintain unchanged sprinting technique when pulling sleds with <10%BM,
but you may also not improve your mechanical effectiveness and horizontal force
output; hence, your performance is also unchanged. So if your running technique
is acutely altered during the VHS sessions, but your running mechanics longitudinally
improve following training, the net advantage in terms of mechanical
effectiveness and performance is pretty clear.

Note that we also believe that training with
light loads (eg 10%BM) may be effective, but to develop other parts of the
sprint mechanical profile. In particular, we think that it may be useful to
train the ability to produce high amounts of horizontal GRF at higher running
speeds, which corresponds to the right part of the sprint force-velocity profile.
We have several studies that will tackle these areas – stay posted!

Conclusion

People who see the topic as “light loads are
useful to improve sprint mechanics and performance” versus “very heavy loads
are better than light loads” are missing the point, in our opinion. This is a
shortcut, and an over simplified ‘black and white’ view of things. The best
answer (sorry for that…) to the question “is loaded sled training useful, and
with what load” is: IT DEPENDS.

It all depends on the goal of training (e.g.
the sport, aim of periodization), the athlete’s individual mechanical profile
and the specific weaknesses. If maximal horizontal force/power output (and
especially the ratio of force) is to be developed, then YES, VHS is useful, and
light loads are likely not or less effective for your immediate goals. If the
needs of the athlete are on the force output at higher speeds, then YES,
lighter loads may be interesting and useful. Our own practice shows that within
a team/group context, some athletes will clearly show different sprint
mechanical profiles, different levels of effectiveness, and so different
training needs. So the solution is not “sleds or not sleds” nor “VHS or not
VHS”, it all depends on what you want to improve. Sprint performance is not
just “being slow vs being fast”.

Determine the important factors for your own
sporting context, each athlete’s mechanical sprint profile, compare athletes,
monitor over time and then decide on an individual basis (who needs what) the
best way to target each athlete’s needs.